MHA is the hydroxy analog of the essential amino acid methionine in racemic form and is, like the latter, an important additive in animal nutrition, especially in poultry raising, but also in many other areas. In addition, MHA has also been used pharmaceutically in the form of its calcium salt to treat renal insufficiency as a substitute for dialysis treatment or to supplement dialysis, as is indicated e.g. in DE-OS 25 31 299.
MHA is usually used in animal nutrition in the form of its aqueous concentrates which also contain, in addition to the monomer, a certain amount of oligomers, primarily the di- and trimeric linear ester acids. The content of these oligomers is a function of the production conditions and also of the concentration selected. However, it is desirable to keep their percentage as low as possible on account of their lower nutritive efficiency of action in comparison to monomeric MHA and on account of the unfavorable influence on the flow properties as a consequence of the elevation of the viscosity. Commercial formulations have, at a total concentration of 88-90% by weight, up to 24% by weight, corresponding to approximately 27 molar %, in sum of oligomers, corresponding to a monomer/oligomer/ratio of approximately 3:1. ##STR1##
The synthesis pathway which is exclusively used industrially starts from methylmercaptopropionaldehyde (MMP), which is converted by HCN addition into the corresponding cyanohydrin (MMP-CH), which is then, catalyzed with sulfuric acid, hydrolyzed to the hydroxy acid. Starting from MHA hydrolyzate, which also contains appropriate amounts of water and ammonium hydrogen sulfate, there are various isolation methods for the valuable product MHA, which are described in DE-OS 19524054 and in DE Patent 44 28 608 in summary form.
These methods include either solvent extraction or precipitation steps or a combination of both for MHA separation from the salt co-produced. Corresponding MHA-containing solutions are concentrated by evaporation in each of these processes. The production of an amount of up to 27 molar % MHA dimers and MHA oligomers must be accepted therewith.
The only way to keep the undesired oligomer components relatively low and to produce a high concentration of MHA almost free of water with a low amount of oligomers (below 10 molar %) is by means of the protective method of evaporation of an organic MHA extraction solution described in DE-OS 19524054. However, the concentration of oligomers rises after several months of storage to over 50 molar %.
A decrease of the oligomer content in the equilibrium to a maximum of 20 molar % succeeds by dilution with water and additionally mixing with methionine or ammonia, which converts a corresponding amount of the MHA into the MHA ammonium salt. A further decrease of the still-present amount of oligomers, which are less effective from the standpoint of nutrition, in an industrial MHA product thus is still desirable.
Furthermore, the products which can be produced according to DE-OS 19524054 as well as the previously available commercial products have an intense brown color with iodine color indices (IFZ) of 20 to over 300 which can be traced back to the oligomeric component and/or also to impurities which are not specifically known. A colorless product was not able to be industrially produced in the past even though this must be characterized as desirable in the sense of a stable, quality improvement of the product.
Distillation of the raw material, which is a known purification method, is discouraged by the high boiling point of MHA of approximately 170.degree. C. at 3 hPa and by the pronounced tendency to autocatalytic oligomer formation. Experiments in a vacuum distillation apparatus resulted, as expected, on account of the formation of MHA oligomer and polymer products, in a low yield of approximately 0.2% of the theoretical yield of distilled MHA. The main portion remained as a dark-brown and viscous material in the distillation bottom (see also Reference Example 3).